Corollary discharge is activity in the nervous system whereby one neuron or pattern of activity causes an efferent discharge. This efferent discharge goes to another area of the CNS, or back to itself, to tell it what is going on. For example, there may be an output from the Central Pattern Generator (CPG) for walking that triggers a corollary discharge (CD) activity that causes inhibition to the quadriceps (to stop hip flexion) and that also sends signal to other parts of the CNS to tell it what is going on. CD is different than sensory input in that it is generated from within the nervous system and is not dependent on sensory input; but also in that it does not give sensory feedback (such as proprioception) but rather is activity output that informs and influences the nervous system in a way different that sensory input.

In terms of how CD helps explain aspect of behavior, I have delineated 4 possibles ways that I will present and discuss below.

1. CD provides an additional explaination for a way to get activity in the CNS other than through sensory input or autonomously. Through CD the nervous system has the capacity to facilitate or inhibit. For example, if a given CPG turns on (either from sensory input or autonomously), the CD pattern that arises from that activity might inhibit the CPG from further discharge. This may be what is occurring with the crayfish swimmerette activity when it is activated by a CPG but then inhibited momentarily by the CD from that CPG. Similarly, this may be what is occurring in human walking patterns when the CPG for the Quads is initiated during the swing and stance phase of gait and then inhibited to allow the hamstrings activity to occur.

2. A second way that CD explains behavior is that it allows a motor symphony to be enacted without there being a central command center. In other words, a motor symphony can be initiated and than carried out at subcortical level without ongoing commands from the cortex. For example, a CPG for walking can be initiated based on the desire to get somewhere(cortical activation), but then enacted at a subcortical level through CPG and CD which provides ongoing information about the motor symphony and which provides a mechanism for coordination of the motor symphony (ie: balance of facilitation and inhibition).

3. CD affects the interpretation of sensory input and is an active component of our perception of reality. This is best explained by Helmholtz's observation regarding eye movements where when the eye is moved passively by pulling at the corner of our eye, the images we see is interpreted to be moving and that the eye is still because we are not getting information from the eye that the eye is moving (ie: there is a lack of CD). Whereas, when we actively move our eyes across a visual field the CD activity tells us that our eye is moving and therefore the visual image is still. The CD from the eye muscles, therefore, affected how we interpreted the sensory input from our visual field. (this concept brings up a question for me: what is the difference between CD and proprioceptive input? When this question was posed in class you responded that CD is from within the CNS whereas sensory input is from without the CNS. But isn't proprioception from "within"? and if so, what distinguish the feedback that we get from proprioception from CD?).

A second example of how CD affects interpretation of sensory input is the phantom limb experience. Melzack suggests that even in the absence of the limb (and therefore in the absence of sensory input), neural networks that continuously generate a characteristic pattern of impulses about the body continue to fire. He calls these neurosignatures. In the absence of sensory input, according to Melzack, cells in the CNS become more active and...."the brain's intrinsic mechanism transform that neuronal activity into meaningful experiences" p 123. In other words, the brain interprets the arm as being there and perhaps as moving. However, as we discussed in class, there is a dissonance created between the perceived sensory input and the expectation of what the sensory input should be from CD pattern. thus, although the individual seems to "feel" the limb because of neursignature patterns that continue to fire, they do not get the typical CD pattern that would emerge when the arm moved. Hence, a dissonance is created and may cause pain.

4. Finally, CD explains one aspect of the capacity for choice in organisms, Albeit at a very primative level. For example, in the pleurobrancha, although a stimulus to the proboscis will usually elicit a withdraw response, when the P. is chewing the stimulus does not elicit withdraw because of a CD pattern from the chewing CPG inhibits the withdraw reaction. Does this account for choice? I feel that in some respects it may, while in other, more complex behaviors it may not. For example, is it possible that a baby, when presented with a novel and interesting toy may mobilize oneself to get the toy and play with it in one instance(ie: when it feels comfortable with its surroundings and the people in it and feels free to explore); and inhibit that behavior in another situation (ie: when an unfamiliar babysitter is with the child and they feel inhibited to explore)? Is this choice a result of a CD pattern that inhibits exploration in the child????? Or is the supression of the drive to explore, a much more complex "I function" interaction of limbic system cues and cortical cues which may result in the baby choosing not to explore?

I will look forward to our discussion on purposeful behavior to add further dimension to this discussion.

Nice summary, with some interesting/appropriate questions/extensions. Proprioceptive is "from within" in one sense, that the signals originate from within the organism rather than from outside the organism. But, they originate from outside the NERVOUS SYSTEM, ie they originate in sensory receptors, unlike CD signals which originate from within the nervous system. That distinction clear? Is worth making, not only to counter the impression that brain activity is dependent on outside input but also because much "proprioceptive" information actually comes not from "enteroreceptors" (like muscle spindles) but also from "exteroreceptors" (like the eyes). Yes, indeed, the responses of a baby under some circumstances and not others may reflect CD's (as may similar observations on adults). Which the, of course, raises your question: what is the role of an "I-function" (you're not prepared to attribute "purposeful" behavior to negative feedback circuits?). PG